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Auke Bay Laboratories (ABL)

Workshop on Top-Down Control of Pacific Herring (Clupea pallasii) in the Gulf of Alaska

The Nutritional Ecology Lab hosted a workshop at the Ted Stevens Marine Research Institute in Juneau, Alaska, on 12 September 2011 to discuss "Top-Down Control of Herring (Clupea pallasii) in the Gulf of Alaska." The workshop marks the completion of several multi-year collaborative studies funded by the Exxon Valdez Oil Spill Trustee Council to weigh the evidence for top-down control of herring in Prince William Sound.

The objective of the workshop was to synthesize the results from these studies that quantify predation impact on herring and to weigh the evidence for top-down control, particularly in Prince William Sound. The product of the workshop will be the collection of manuscripts from these studies in a special issue of a journal with an overarching summary framed from the workshop discussion. A summary of the workshop follows.

Humpback whales are one of the most conspicuous herring predators in the Gulf of Alaska (GOA), and they are increasing in abundance at a rapid rate (4%-7%/year). Since the cessation of whaling in 1966, humpback abundance has recovered and now exceeds that of pre-whaling days. We quantified the significance of humpback whale predation on herring in winter when herring form large aggregations and whales are building their energy reserves for their annual breeding migrations.

Whale predation was compared amongst three GOA herring stocks, including depressed herring stocks in Prince William Sound and Lynn Canal and a robust stock in Sitka Sound. We determined that humpbacks had the greatest impact on herring in Prince William Sound, where whales were most abundant and remained longer into the winter. Observations of feeding whales and isotopic and fatty acid analysis of blubber indicated that in Prince William Sound, whales foraged on herring for a longer period of time into winter in contrast to Sitka where the greatest proportion of whales were feeding on krill. Prolonged foraging on herring in Prince William Sound may be due to absence of alternate prey or a preference for herring.

Bioenergetic modeling efforts reveal that humpbacks remove the highest proportion of herring biomass in depressed areas, in quantities similar to that of a commercial fishery (more than 20% in Prince William Sound and Lynn Canal versus 1% in Sitka). This equates to approximately 3,500 metric tons (t) of herring consumed over winter in Prince William Sound, 900 t in Sitka, and 600 t in Lynn Canal. Foraging by humpback whales may disrupt herring school formation at depth, facilitating foraging by other air-breathing predators including Steller sea lions and sea birds.

Similar surveys and bioenergetic modeling efforts were conducted to quantify winter predation on herring by seabirds in Prince William Sound from 1990 to 2007. Eighteen species of marine birds were identified as herring predators, with consumption estimates being driven by common murres that were more abundant than other species by 1-2 orders of magnitude. On average, seabirds consumed approximately 2,400 t of herring per winter.

Modeling results indicate that biomass of juvenile herring consumed may be twice that of adults. This represents an average removal of 6% of the adult herring biomass, and as much as 10% in years with low herring biomass and high murre abundance. Evidence of top-down control by seabirds was observed using a lag analyses in which seabird consumption of adult herring was negatively correlated with miles of spawn observed the following year.

Similarly, a retrospective analysis was conducted to look for relationships in humpback whale and herring abundances in Prince William Sound over time. Incorporation of whale-derived mortalities in the herring age-structured assessment (ASA) model revealed a relationship between whale abundance and that of older herring (ages 5+), but not younger age classes (ages 3-4).

Though the modeling results indicated that humpback predation could be substantially impacting the biomass of older herring, removal of whale predation from the model did not substantially affect the resulting biomass of herring. This suggests that whale-derived mortality was less important than recruitment on biomass. Further, the whale-adjusted ASA model indicated that whale-induced mortalities increased at the same time periodic recruitment became significantly hindered.

To examine the potential for impaired recruitment to structure the Prince William Sound herring stock, two mechanisms of recruitment were compared between three Gulf of Alaska herring stocks over three winters, including reproductive investment of adults and juvenile mortality. We examined the potential influence of winter on Pacific herring by contrasting the winter energy consumption of juveniles and adults from three stocks around the Gulf of Alaska. In addition, we performed laboratory studies aimed at understanding how metabolic rates scale with temperature and estimating the energy cost associated with disease infection.

These analyses indicated that the sensitivity of juvenile herring to winter varies spatially as a result of differences in food availability and sources of energetic costs. A direct consequence of this variation is that juveniles in different locations begin spring in different nutritional states. In addition, we determined that juvenile herring undergo compensatory growth in spring and the degree of compensation depends on the nutritional status at the end of winter.

Laboratory studies demonstrate that the presence of disease in juvenile herring can impair compensation by imposing a metabolic cost. These factors likely interact to influence recruitment of juvenile herring into spawning populations and suggest recruitment models in stock assessments can be improved by monitoring juvenile condition and abundance. In contrast, winter appears to have less influence on the amount of energy allocated to gonadic tissues in adult herring.